This invention relates to control systems for hydraulically-operated construction machinery, and more particularly it is concerned with a control system for hydraulically-operated construction machinery, such as a hydraulic excavator of the crawler or wheel type, equipped with a prime mover and at least one variable displacement hydraulic pump driven by the prime mover to operate desired working elements through the respective actuators.
One of the typical hydraulically-operated construction machinery is a hydraulic excavator which generally comprises a prime mover or an engine, means for controlling the revolution number of the engine such as a fuel injection device equipped with an engine lever operated by the operator in the operator's cab and a governor connected to the engine lever, at least one variable displacement pump driven by the engine, means for controlling the displacement volume of the pump or a regulator, a pluraly of actuators driven by a hydraulic fluid delivered from the hydraulic pump and a plurality of working elements driven by the respective actuators. Generally, the working elements include left and right travel devices, a swing and front attachments, such as a boom, an arm and a bucket.
In the hydraulic excavator of the aforesaid construction, a control lever is provided for each one of the working elements and operated by the operator to actuate a directional control valve mounted between the hydraulic pump and the actuator for the respective working element to supply a hydraulic fluid from the hydraulic pump being driven by the engine to the corresponding actuator to drive the same, to thereby drive the respective working element for operation. In this way, the left and right travel devices are operated for travelling, the swing is operated for rotation and the various front attachments are suitably operated for performing a digging operation.
In a hydraulic excavator of the prior art, the maximum value of the revolution number of the engine controlled by the fuel injection device is generally constant and the maximum value of the hydraulic pump controlled by the regulator is constant. For example, in a case where the pump is swash-plate pump, the maximum value of the tilting angle of the swash plate is generally constant. The constant maximum revolution number and maximum displacement volume are decided so as to be able to obtain a desired maximum discharge rate at the maximum revolution number of the engine and at the same time a desired pump consumption horsepower at the maximum revolution number of the engine.
The constant maximum revolution number and maximum displacement volume are decided in such a manner that the desired maximum pump flow rate and the desired engine horsepower characteristic can be obtained. In other words, the maximum pump flow rate of the hydraulic excavator of this construction is decided by the product of the maximum revolution number and maximum displacement volume and its engine horsepower characteristic is decided by the maximum revolution number that has been set.
Meanwhile the working elements of the hydraulic excavator are each desired to do work such as travelling, rotation or digging, which is peculiar to the respective working element. However, a mode of operation of each working element, such as working speed or work volume, may greatly vary from time to time. In the travel device, for example, there are high speed and low speed travellings and in the front attachments, there are heavy digging operations in which work volume to be handled is high, light digging operations in which work volume to be handled is relatively small and fine operations in which the working speed is very low.
One of the problems encountered in the hydraulic excavator of the prior art is that, since the maximum revolution number of the engine and the maximum displacement volume of the hydraulic pump are constant as described hereinabove, difficulty is experienced for the operator to place the machine in an operation mode in which the operation speed and work volume of each working element are exactly as intended.
For example, if the maximum revolution number of the engine and the maximum displacement volume of the hydraulic pump are so that the maximum discharge rate of the pump becomes large by placing emphasis on high-speed travelling, it is possible to let the travel devices operate at a high speed but the flow rate of fluid would be too high for other working elements. When it is required to operate one of the front attachments in fine operation, it would be quite difficult to attain the fine operation and the operability of the front attachment would be lowered. Conversely, if an emphasis is placed on performing fine operations and the maximum flow rate of fluid from the pump is set at not so high level, then it would be possible to perform a fine operation satisfactorily but it would be impossible to let the excavator travel at high speed because the flow rate of fluid supplied to travel motors would be restricted.
When the maximum revolution number and the maximum displacement volume are set with an eye to achieving an engine horsepower characteristic capable of providing a high pump consumption horsepower and optimizing the maximum fluid flow rate from the pump by placing emphasis on the heavy digging, it is possible for the excavator to perform heavy digging operations at a high working speed. However, if the excavator performs light digging operations at the same working speed, the engine horsepower would partly be wasted and fuel would be unnecessary consumed, thereby increasing noises produced by the engine.
Another typical hydraulically-operated construction machinery is a hydraulic excavator of the wheel type which is substantially similar in construction to the hydraulic excavator of the crawler type except that it is equipped with pneumatic tires driven by the travel motors for rotation. In the hydraulic excavator of the wheel type, the maximum revolution number of the engine and the maximum displacement volume of the hydraulic pump are set constant as is the case with the hydraulic excavator of the crawler type.
The maximum speed of the hydraulic excavator of the wheel type is set in many cases at a relatively high value owing to the fact that this type of excavator is allowed to travel not only at the site of work but also on roads in general. In Japan, for example, the maximum speed of this type of excavator is set by law at 35 km/h. Thus the hydraulic excavator of the wheel type is required to be able to travel at a maximum speed of 35 km/h. Because roads include flat and sloping ones, it is desired that the excavator is capable of travelling at the maximum speed of 35 km/h set by law.
Thus the hydraulic excavator of the wheel type is distinct from the hydraulic excavator of the crawler type in that it has a relatively high travel speed and that it is preferably able to travel at the maximum speed of 35 km/h set by law even when it moves uphill on a sloping road where a high load is applied thereto. This distinction gives rise to problems peculiar to the hydraulic excavator of the wheel type with regard to providing a working mode intended by the operator.
Namely, if the engine horsepower characteristic is set so as to be able to obtain a high pump consumption horsepower and the maximum revolution number of the engine and the maximum displacement volume of the hydraulic pump are set so as to be able to obtain high flow rate by placing emphasis on the ability of the excavator to move uphill on a sloping road where a high load is applied thereto the excavator would suffer disadvantages in being high in fuel consumption and noise production when travelling on flat roads or performing light digging operations under a low load, because these operations require a lower pump consumption horsepower than moving uphill on a sloping road. Conversely, if the maximum revolution number of the engine and the maximum displacement volume of the hydraulic pump are set by placing emphasis on travelling on flat roads or performing ordinary digging operations, then it would be impossible to obtain necessary horsepower for moving uphill on sloping roads, making it impossible to travel at the maximum speed of 35 km/h set by law.
Meanwhile, Japanese Patent Unexamined Publication No. 135341/83 (Patent Application No. 16349/82) laid open to public inspection in Japan on Aug. 11, 1983 discloses a control system for hydraulically-operated construction machinery which is provided with means for sensing the working conditions of the actuators so that, when an actuator tending to have a high load applied thereto is sensed to be in a working condition, the revolution number of the engine is increased and the displacement volume of the hydraulic pump is reduced and, when an actuator tending to have a low load applied thereto is sensed to be in a working condition, the revolution number of the engine is reduced and the displacement volume of the hydraulic pump is increased. Thus a sudden change in the flow rate of fluid from the pump that might be caused by a change in the load is prevented to conserve energy and improve operability.
As described hereinabove, the operation mode of each working element, such as working speed and work volume, varies and hence a load applied to each actuator varies. The control system of the prior art disclosed in the publication for the hydraulic excavator has been unable to cope with such variations in the working mode. In other words, the control system of the prior art has not solved the problem raised when light digging operations are performed while the maximum value of the revolution number of the engine and the displacement volume of the hydraulic pump are set by placing emphasis on performing heavy digging operations satisfactorily, and the problem raised when the excavator travels on flat roads while they are set by placing emphasis on the excavator travelling uphill on sloping roads. Similar problems are raised when fine operations are performed after the maximum value of the revolution number of the engine and the displacement volume of the hydraulic pump are set by placing emphasis on the excavator being able to travel at high speed, because it is essentially impossible to set the maximum value of the number of revolutions of the engine and the displacement volume of the hydraulic pumps with an eye to fine operations which have a low incidence. Thus the hydraulically-operated excavator having the control system described hereinabove would have essentially the same disadvantage as conventional hydraulic excavators in that it is impossible to provide each working element with an operation mode which is desired by the operator.